1. Field of the Invention
The present invention relates generally to spacecraft and, more particularly, to spacecraft solar arrays.
2. Description of the Related Art
Spacecraft typically carry solar cells as a primary energy source with rechargable batteries providing energy storage for eclipse operations. The solar cells are positioned on the spacecraft so that they are exposed to solar radiation.
On spinning spacecraft, solar cells are generally arranged about the outside of a spinning spacecraft body. Accordingly, only a fraction of the cells are exposed to solar radiation at any instant in time. On body-stabilized spacecraft, in contrast, solar cells are typically arranged in planar arrays and carried on solar wings which extend from opposite sides of a spacecraft body. Preferably, the solar wings rotate to keep them as orthogonal to the solar radiation as possible. Because the solar wings can be quite long in their deployed configuration, they generally are formed of a plurality of planar solar panels which are coupled together in an accordion arrangement so that they can be collapsed to a smaller stowed configuration for spacecraft launch.
The number of solar cells that must be carried by a spacecraft is a function of the anticipated spacecraft power demand and the efficiency of the solar cells. Although high-efficiency solar cells reduce the number of cells required by a specific spacecraft, they are quite expensive. Because weight and weight-related costs also increase with the number of solar cells, there is a considerable incentive to reduce the quantity of solar cells that a spacecraft must carry.
Accordingly, efforts have been extended to concentrate solar radiation upon solar cells with reflectors that are positioned to reflect radiation upon the cells. Solar radiation that would otherwise have passed by a solar wing is thus redirected to be incident upon the solar cells. Although energy conversion efficiency of this reflected radiation is typically less than that of direct radiation because of a lesser angle of incidence, the number of spacecraft solar cells can still be significantly reduced with consequent savings in spacecraft weight and cost.
A variety of reflector systems have been proposed. In an exemplary system of U.S. Pat. No. 4,282,394, reflector arms are carried on both inboard and outboard frames. Each of the reflector arms is formed of a plurality of hinged arm sections and each arm section of the inboard frame carries a reflective sheet that is wound on a spring-biased roll. An end of each sheet is attached to a respective arm section on the outboard frame.
During deployment, an extensible shaft moves the outboard frame away from the inboard frame and each reflective sheet is unrolled to reflect solar radiation onto solar cells. Although this reflector system concentrates solar radiation, its complex structure (e.g., hinged arms, inboard and outboard frames and extensible shaft) significantly contributes to spacecraft weight and cost.
In a Naval Research Laboratory design, a single thin-film reflector spans a plurality of solar panels that are coupled together in an accordion arrangement. Each thin-film reflector is carried with tension springs between a pair of rotatable booms. Because the reflector film is held in tension, its edges assume a catenary shape. In order to fold the solar panels into a stowed position, the booms rotate to lie alongside the panels and the thin-film reflector is rolled (e.g., from the reflector center) so that it lies parallel to the booms. Although this reflector system is potentially lighter and simpler than the system described above, it still involves numerous mechanical parts (e.g., booms, cables and pulleys) which have significant weight and degrade reliability.
Other reflector systems are described in U.S. patent application Ser. No. 08/081,909, filed Jun. 18, 1993 and now abandoned (as a continuation of application Ser. No. 07/802,972, filed Dec. 6, 1991 and now abandoned), titled "Augmented Solar Array with Dual Purpose Reflectors" and assigned to Hughes Electronics, the assignee of the present invention. In an exemplary system, a reflector is formed from a reflective material (e.g., an aluminized polyimide film) that is carried by a peripheral frame or affixed over a ribbed structure or a thin metal sheet. Each reflector is coupled to a respective solar panel by a hinge mechanism. Prior to spacecraft launch, the reflector is rotated to lie over the solar cell face of its respective solar panel. After launch, the hinge mechanism rotates the reflector to a position in which it forms a deployment angle with the solar cell face. In an exemplary hinge mechanism, a hinge spring urges the reflector to rotate away from the solar cell face. The hinge mechanism includes a stop member which halts this rotation when the reflector reaches the deployment angle.
In another reflector system embodiment, reflectors are fabricated by suspending a reflective film between a pair of flexible rods that are rigidly coupled to a solar panel. The rods are typically tethered such that the reflectors lie over the solar cell face prior to spacecraft launch. Deployment is effected by untethering which allows the rods to whip directly to a position in which the reflective film forms a deployed angle with the panel.
Other conventional reflector systems have added secondary reflectors at the ends of solar wings so as to redirect nonorthogonal solar radiation onto solar panels.
Although these various reflector systems redirect solar radiation to solar panels, their storage of reflectors over the solar cell face causes the reflectors to block the use of any of the solar panels during any period (e.g., a transfer orbit) in which the panels are in their stowed configuration.
In addition, these reflector systems fail to provide structures that insure that one of the reflectors does not damage and degrade another of the reflectors during reflector deployment.
Typically, restraint members (e.g., explosive bolts) maintain solar panels in their stored configuration prior to deployment. Solar reflectors which fold over respective solar panels must define apertures to permit passage of the restraint members. These conventional reflector systems fail to address the loss of reflection caused by the apertures.